If your score is in the low 700/690 range, you should heed drbond's advice and proceed very cautiously about "churning" cards. There is a huge difference between someone has a high 700 score and someone has a low 700 score.

You would be better off in the long run, to try to figure out WHY you have such a low score (being young is not necessary the correct reason), before you try so hard and poke around to "perfect the art of churning". There is no hard and fast rules because as drbond already so kindly hinted, there are so many variables involved in the processing of each application, no one can tell you exactly what would work and what wouldn't.